1. Field of the Invention
The present invention relates to a method of reducing inter-subcarrier interference in multicarrier communication systems and a receiver using it, and more particularly to a receiver for use in multicarrier communication systems to suppress inter-subcarrier interference.
2. Description of the Related Art
A conventional transmitter for use in multicarrier communication systems is configured of an encoder 31, a demultiplexer 32, lowpass filters (LPFs) 33 and 34, digital-to-analog (D/A) converters 35 and 36, multipliers 37 and 38, a synthesizer 39, a transmitting amplifier 40 and a transmitting antenna 41 as shown in FIG. 2.
A receiver for use in multicarrier communication systems is configured of a receiving antenna 201, a receiving amplifier 202, multipliers 203 and 204, lowpass filters (LPFs) 205 and 206, analog-to-digital (A/D) converters 207 and 208, a multiplexer 209 and a decoder 210 as shown in FIG. 1. The transmitter and the receiver mentioned above are supposed to use two subcarriers.
First in the transmitter, input signals are encoded by the encoder 31, and demultiplexed by the demultiplexer 32 into two sequences. Each of the sequences into which the signals have been demultiplexed by the demultiplexer 32 is bandwidth-limited by the lowpass filters 33 and 34.
The signals bandwidth-limited by the lowpass filters 33 and 34 are converted into analog signals by the D/A converters 35 and 36, and modulated by the multipliers 37 and 38 with respect to subcarriers. The signals modulated with respect to the subcarriers are mixed by the synthesizer 39, amplified by the transmitting amplifier 40, and transmitted from the transmitting antenna 41.
On the other hand in the receiver, receive signals received by the receiving antenna 201 are amplified by the receiving amplifier 202, subjected to quadrature demodulation by the multipliers 203 and 204 with respect to the respective subcarriers, and band-limited by the lowpass filters 205 and 206.
The receive signals band-limited by the lowpass filters 205 and 206 are converted into digital signals by the A/D converters 207 and 208 at a sampling frequency fs. The receive signals converted into digital signals are multiplexed into a single sequence by the multiplexer 209, and decoded by the decoder 210 into output signals.
In the multicarrier communication system, to prevent signal interference between subcarriers, the frequency band of each subcarrier is limited by the lowpass filters 33 and 34, and sufficient frequency spacing is ensured between the subcarriers to prevent the frequency bands of the subcarriers from overlapping each other or, in the orthogonal frequency division multiplexing (OFDM) system, the frequency bands of subcarriers are allowed to overlap each other by using mutually orthogonal subcarriers.
However, in the receiver of the conventional multicarrier communication system described above, if the frequency bands of subcarriers overlap each other as shown in FIG. 5, signal interference occurs between the subcarriers, inviting a deterioration in reception performance.
Therefore, in the receiver of the conventional multicarrier communication system, the band limiting filters should have sufficient attenuation characteristic outside the bands, and the spacing between the subcarriers should be sufficiently wide.
Or in the OFDM system referred to above, as it uses mutually orthogonal subcarriers, the orthogonality cannot be maintained once waveforms are distorted on multiple paths, inviting signal interference between the subcarriers.